Optical element unit

ABSTRACT

An optical element unit is provided comprising an optical element group for projecting light along an optical axis of the optical element group and a housing having an inner housing part partly defining a first space and a light passageway between the inner housing part and a second space. The inner housing part receives the optical element group. The optical element group comprises an ultimate optical element located in the region of the light passageway. A load-relieving device is provided adjacent to the ultimate optical element, the load relieving device partly defining the first space and the second space and at least partly relieving the ultimate optical element from loads resulting from pressure differences between the first space.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is claiming priority of U.S. Provisional PatentApplication No. 60/659,845, filed Mar. 9, 2005, the content of which isherein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical element units used in exposureprocesses, in particular to optical element units of microlithographysystems. It further relates to optical exposure apparatuses comprisingsuch optical element units. Furthermore, it relates to methods ofmanufacturing such optical element units. The invention may be used inthe context of photolithography processes for fabricatingmicroelectronic devices, in particular semiconductor devices, or in thecontext of fabricating devices, such as masks or reticles, used duringsuch photolithography processes.

2. Description of the Related Art

Typically, the optical systems used in the context of fabricatingmicroelectronic devices such as semiconductor devices comprise aplurality of optical elements, such as lenses and mirrors etc., in thelight path of the optical system. Those optical elements usuallycooperate in an exposure process to transfer an image formed on a mask,reticle or the like onto a substrate such as a wafer. The opticalelements are usually combined in one or more functionally distinctoptical element groups. These distinct optical element groups may beheld by distinct optical exposure units. Such optical exposure units areoften built from a stack of optical element modules holding one or moreoptical elements. These optical element modules usually comprise anexternal generally ring shaped support device supporting one or moreoptical element holders each, in turn, holding an optical element.

Optical element groups comprising at least mainly refractive opticalelements, such as lenses, mostly have a straight common axis of symmetryof the optical elements usually referred to as the optical axis.Moreover, the optical exposure units holding such optical element groupsoften have an elongated substantially tubular design due to which theyare typically referred to as lens barrels.

Due to the ongoing miniaturization of semiconductor devices there is apermanent need for enhanced resolution of the optical systems used forfabricating those semiconductor devices. This need for enhancedresolution obviously pushes the need for an increased numerical apertureand increased imaging accuracy of the optical system.

Furthermore, to reliably obtain high-quality semiconductor devices it isnot only necessary to provide an optical system showing a high degree ofimaging accuracy. It is also necessary to maintain such a high degree ofaccuracy throughout the entire exposure process and over the lifetime ofthe system. As a consequence, the optical elements of such an opticalsystem must be supported in a defined manner in order to maintain apredetermined spatial relationship between the optical elements toprovide a high quality exposure process.

Depending on the wavelength of the light used in such exposureprocesses, it is often necessary to maintain a gas atmosphere within therespective optical element units in order to reduce absorption effects.During the exposure process pressure differences between the gasatmosphere within the optical element unit and the environmentsurrounding the optical element unit may occur. These pressuredifferences may cause a position variation of the ultimate opticalelement, typically a so called last lens element, located near or at theexit end of the optical element unit and typically separating theinterior of the optical element unit from the surrounding environment.

Depending on the design of the respective last lens element, inparticular on the optical sensitivity of the last lens element toposition variations, such position variations may have a considerableadverse effect on the imaging accuracy and, thus, on the overall qualityof the exposure process. To largely avoid these effects, currently,considerable effort is necessary to hold such an ultimate opticalelement in a manner which is as rigid as possible. This is particularlycomplicated if, for other purposes, the ultimate optical element has tobe held in an adjustable manner.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to, at least to someextent, overcome the above disadvantages and to provide good and longterm reliable imaging properties of an optical system used in anexposure process.

It is a further object of the present invention to maintain imagingaccuracy of an optical system used in an exposure process by reducingthe sensitivity of the optical system to pressure differences between anatmosphere within an optical element unit of the optical system and theenvironment surrounding the optical element unit.

These objects are achieved according to the present invention which isbased on the teaching that a reduction of the sensitivity of the opticalsystem to pressure differences between an atmosphere within an opticalelement unit of the optical system and the environment surrounding theoptical element unit may be achieved when the optical elementpotentially subjected to loads resulting from such pressure differencesis at least partially relieved from such loads resulting from suchpressure differences.

Thus, according to a first aspect of the present invention there isprovided an optical element unit comprising an optical element group forprojecting light along an optical axis of the optical element group anda housing having an inner housing part partly defining a first space anda light passageway between the inner housing part and a second space.The inner housing part receives the optical element group. The opticalelement group comprises an ultimate optical element located in theregion of the light passageway and being part of an ultimate opticalelement arrangement. A load relieving device is provided adjacent to theultimate optical element arrangement, the load relieving device partlydefining the first space and the second space and at least partlyrelieving the ultimate optical element arrangement from loads resultingfrom pressure differences between the first space and the second space.

According to a second aspect of the present invention there is providedan optical element unit comprising an optical element group forprojecting light along an optical axis of the optical element group, ahousing and a cover device. The housing has an inner housing part partlydefining a first space and a light passageway between the first spaceand a second space, the inner housing part receiving the optical elementgroup. The optical element group comprises an ultimate optical elementlocated in the region of the light passageway and having an opticallyused first surface. The cover device is mounted to the housing andextends between the housing and the first surface of the ultimateoptical element. Furthermore, the cover device leaves a first surfacepart of the first surface of the ultimate optical element uncoveredwhile it covers a second surface part of the first surface of theultimate optical element.

According to a third aspect of the present invention there is providedan optical element unit comprising an optical element group forprojecting light along an optical axis of the optical element group, ahousing and a sealing device. The housing has an inner housing partpartly defining a first space and a light passageway between the firstspace and a second space, the inner housing part receiving the opticalelement group. The optical element group comprises an ultimate opticalelement located in the region of the light passageway and a furtheroptical element. The sealing device separates the first space from thesecond space. The sealing device is located between the further opticalelement and the ultimate optical element such that the ultimate opticalelement is located within the second space. Furthermore, the sealingdevice has least a first translucent section.

According to a fourth aspect of the present invention there is providedan optical exposure apparatus for transferring an image of a patternformed on a mask onto a substrate comprising a light path, a masklocation located within the light path and receiving the mask, asubstrate location located at an end of the light path and receiving thesubstrate, and an optical element unit according to the presentinvention located within the light path between the mask location andthe and the substrate location.

According to a fifth aspect of the present invention there is provided amethod of holding an optical element, comprising providing a housinghaving an inner housing part partly defining a first space and a lightpassageway between the inner housing part and a second space, a firstpressure prevailing in the first space and a second pressure prevailingin the second space; providing and holding an optical element in theregion of the light passageway, the optical element mainly extending ina first plane, a first part of the optical element partly defining thefirst space and a second part of the optical element partly defining thesecond space; and separating the first space and the second space suchthat a projection of the first part of the optical element onto thefirst plane is smaller than a projection of the second part of theoptical element onto the first plane.

According to a sixth aspect of the present invention there is provided amethod of holding an optical element, comprising providing a housinghaving an inner housing part partly defining a first space and a lightpassageway between the inner housing part and a second space, a firstpressure prevailing in the first space and a second pressure prevailingin the second space; providing and holding an optical element in theregion of the light passageway; and separating the first space and thesecond space in a pressure tight but at least partly translucent mannersuch that the optical element is entirely located within the secondspace.

Further aspects and embodiments of the present invention will becomeapparent from the dependent claims and the following description ofpreferred embodiments which refers to the appended figures. Allcombinations of the features disclosed, whether explicitly recited inthe claims or not, are within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a preferred embodiment of anoptical exposure apparatus according to the present invention comprisinga preferred embodiment of an optical element unit according to thepresent invention;

FIG. 2 is a schematic sectional representation of a part of the opticalelement unit of the optical exposure apparatus of FIG. 1;

FIG. 3 is a schematic sectional view of the detail III of FIG. 2;

FIG. 4 is a block diagram of a preferred embodiment of a method ofmanufacturing the optical element unit of FIG. 1 including a preferredembodiment of a method of holding an optical element according to thepresent invention;

FIG. 5 is a schematic representation of a further preferred embodimentof an optical exposure apparatus according to the present inventioncomprising a preferred embodiment of an optical element unit accordingto the present invention;

FIG. 6 is a schematic sectional view of a detail of a further preferredembodiment of an optical element unit according to the present inventionused in the optical exposure apparatus of FIG. 5;

FIG. 7 is a schematic representation of a further preferred embodimentof an optical exposure apparatus according to the present inventioncomprising a preferred embodiment of an optical element unit accordingto the present invention;

FIG. 8 is a schematic sectional representation of a part of the opticalelement unit of the optical exposure apparatus of FIG. 7;

FIG. 9 is a schematic sectional view of the detail IX of FIG. 8;

FIG. 10 is a schematic representation of a further preferred embodimentof an optical exposure apparatus according to the present inventioncomprising a preferred embodiment of an optical element unit accordingto the present invention;

FIG. 11 is a schematic sectional representation of a part of the opticalelement unit of the optical exposure apparatus of FIG. 10;

FIG. 12 is a schematic representation of a further preferred embodimentof an optical exposure apparatus according to the present inventioncomprising a preferred embodiment of an optical element unit accordingto the present invention;

FIG. 13 is a schematic sectional representation of a part of the opticalelement unit of the optical exposure apparatus of FIG. 12;

FIG. 14 is a block diagram of a preferred embodiment of a method ofmanufacturing the optical element unit of FIG. 12 including a preferredembodiment of a method of holding an optical element according to thepresent invention;

FIG. 15 is a schematic representation of a further preferred embodimentof an optical exposure apparatus according to the present inventioncomprising a preferred embodiment of an optical element unit accordingto the present invention.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

In the following, a first preferred embodiment of an optical exposureapparatus 1 according to the present invention comprising an opticalprojection system 2 with an optical element unit 3 according to thepresent invention will be described with reference to FIGS. 1 to 3.

The optical exposure apparatus 1 is adapted to transfer an image of apattern formed on a mask 4 onto a substrate 5. To this end, the opticalexposure apparatus 1 comprises an illumination system 6 illuminating themask 4 and the optical element unit 3. The optical element unit 3projects the image of the pattern formed on the mask 4 onto thesubstrate 5, e.g. a wafer or the like.

To this end, the optical element unit 3 holds an optical element group7. This optical element group 7 is held within a housing 3.1 of theoptical element unit 3. The optical element group 7 comprises a numberof optical elements 8 and optical elements 9 and 10, such as lenses,mirrors or the like. These optical elements 8, 9, 10 are aligned alongan optical axis 3.2 of the optical element unit 3.

The optical projection system 2 receives the part of the light pathbetween the mask 4 and the substrate 5. Its optical elements 8, 9, 10cooperate to transfer the image of the pattern formed on the mask 4 ontothe substrate 5 located at the end of the light path. To this end, theoptical elements 8, 9, 10 of the optical element group 7 project thelight received form the illumination system 6 along the optical axis3.2.

The optical element unit 3 is composed of a plurality of optical elementmodules 3.3 and optical element modules 3.4 and 3.5 stacked and tightlyconnected to form the optical exposure unit 3. Each optical elementmodule 3.3, 3.4, 3.5 holds one or more of the optical elements 8, 9, 10,respectively.

FIG. 2 shows a schematic sectional representation of the last threeoptical element modules 3.3, 3.4 and 3.5 of the optical element unit 3.As can be seen in particular from this Figure, the housing 3.1 has aninner housing part 3.6 partly defining a first space 11 and a lightpassageway 3.7 between the inner housing part 3.6 and a second space 12open to the environment surrounding the housing 3.1. While a firstpressure prevails in the first space 11, a second pressure prevails inthe second space 12.

Within this light passageway 3.7, located at the exit end 3.8 of theoptical element unit 3, there is provided an ultimate optical elementarrangement comprising an ultimate optical element in the form of a lastlens element 10 and an ultimate optical element holder in the form of alast lens element holder 13. The last lens element holder 13 is holdingthe last lens element 10 so as to be adjustable in position. This lastlens element 10 partly separates the first space 11 and the second space12 and, thus, partly defines the first space 11 and the second space 12.

The last lens element 10 is a rotationally symmetric lens having a firstaxis of symmetry 10.1 essentially coinciding with the optical axis 3.2of the optical element unit 3. The last lens element 10 mainly extendsin a first plane perpendicular to the first axis of symmetry 10.1, as itis indicated in FIG. 2 by the dashed line 10.2. Furthermore, the lastlens element 10 has a first perpendicular projection onto the firstplane 10.2 which has a first area A1.

The last lens element 10 has a first optical element surface in the formof a first lens surface 10.3 and a second optical element surface in theform of a second lens surface 10.4. The first lens surface 10.3 facestowards the penultimate optical element 9 of the optical element group7. The second lens surface 10.4 faces away from the penultimate opticalelement 9 of the optical element group 7.

The first lens surface 10.3 has a first surface region 10.5 opticallyused during an exposure process. This first surface region 10.5 is ofcircular shape. The outer circumference of the first surface region 10.5is indicated in FIG. 2 by the intersection of the dashed lines 10.6 withthe first lens surface 10.3.

Between the last lens element 10 and the penultimate optical element 9,a cover device in the form of a thin walled cover 14 is provided withinthe inner housing part 3.6. This cover 14 substantially has the form ofa truncated conical shell. At its outer circumference 14.1, the thinwalled cover 14 is mounted to the housing 3.1 in a substantiallypressure tight manner.

A third space 12.1 is defined by the housing 3.1, the last lens elementholder 13, the last lens element and the cover 14. This third space 12.1communicates with the environment on the other side of the last lenselement holder 13 via venting openings 13.1 within the last lens elementholder 13. Thus, the third space 12.1 forms part of the second space 12,the second pressure also prevailing in the third space 12.1.

In this context, it will be appreciated that, with other embodiments ofthe present invention, the venting openings or venting passageways,additionally or alternatively, may also be provided in the housing andthe ultimate optical element, respectively.

The cover 14 extends between the housing 3.1 and the last lens element10. At its inner circumference 14.2, the cover 14 is located immediatelyadjacent to the first lens surface, thereby forming an aperture 14.3 atthis inner circumference 14.2. This aperture 14.3 leaves a certain partof the first lens surface 10.3 uncovered by the cover 14.

The cover 14 is not contacting the last lens element 10 directly, as canbe seen from FIG. 3 in greater detail. In the embodiment shown, thecover 14 contacts the last lens element 10 via a sealing element in theform of a thin sealing membrane 15. This sealing membrane 15 extendsbetween the inner circumference 14.2 of the cover 14 and the last lenselement 10. The sealing membrane 15 is connected to the cover 14 and thelast lens element 10 in a pressure tight manner.

Thus, the first space 11 and the second space 12 are separated by thecover 14, the sealing membrane 15 and the last lens element 10 in apressure tight manner to avoid contamination of the first space 11 withcontaminants from the environment surrounding the housing.

The cover 14 is located such that a first part of the last lens element10 partly defines the first space 11 and that a second part of the lastlens element 10 partly defines the second space 12. The perpendicularprojection of this first part of the last lens element 10 onto the firstplane 10.2 is smaller than the perpendicular projection of this secondpart of the last lens element 10 onto the first plane 10.2.

The cover 14 has the beneficial effect that only a second surface region10.7 of the first lens surface 10.3 is subjected to the first pressureprevailing in the first space 11 while the rest of the lens surface andalso the last lens element holder 13 is subjected to the second pressureprevailing in the second space 12. The outer circumference of thissecond surface region 10.7 is defined by the sealing membrane 15contacting the first lens surface 10.3.

Thus, the force F acting in the direction of the optical axis 3.2 on thelast lens element 10 in the direction of the optical axis 3.2 due topressure differences between the first pressure p₁ in the first space 11and the second pressure p₂ in the second space 12 may be calculated asfollows:

F=A _(ssr)·(p ₁ −p ₂)=A _(ssr) ·dp,  (1)

-   wherein: A_(ssr) is the area of the perpendicular projection of the    second surface region 10.7 onto the first plane 10.2;    -   dp is the pressure difference between the first pressure p₁ in        the first space 11 and the second pressure p₂ in the second        space 12.

Consequently, a variation Δdp in the pressure difference dp will resultin a variation ΔF in the force F which calculates as follows:

ΔF=A _(ssr) ·Δdp.  (2)

As becomes apparent from equation (2), compared to a conventional designwithout a cover 14, the cover 14 considerably reduces the variation ΔFin the force F acting on the last lens element 10 due to a variation Δdpin the pressure difference dp between the first space 11 and the secondspace 12.

Thus, compared to a conventional design without a cover 14, at a givenpressure variation Δdp and a given rigidity of the last lens elementholder 13 in the direction of the optical axis 3.2 a lower positionvariation of the last lens element 10 due to this variation Δdp isachieved with the cover 14 according to the invention. Thus, the cover14 with the sealing membrane 15 forms a load relieving device relievingthe last lens element 10 and, thus, the ultimate optical elementarrangement from loads resulting from pressure differences within thefirst space 11 and the second space 12.

As a consequence, to keep certain position variation limits for the lastlens element 10 during operation, less effort is required for therigidity of the last lens element holder 13 in the direction of theoptical axis.

It will be appreciated that, to achieve this beneficial effect accordingto the invention, the aperture 14.3 has a second perpendicularprojection onto the first plane 10.2 with a second area A₂ which issmaller than the first area A₁ of the first perpendicular projection ofthe last lens element 10 onto the first plane 10.2.

To largely take advantage of this beneficial effect without affectingthe optical performance of the optical system, the aperture 14.3 islocated as close as possible to the first surface region 10.5 opticallyused during an exposure process, as it is shown in the FIGS. 2 and 3.Thus, the second area A₂ substantially corresponds to of the thirdperpendicular projection of the first surface region 10.5 onto the firstplane 10.2.

In other words, the aperture 14.3 is located as close as possible to thefirst surface region 10.5 optically used during an exposure process suchas to leave uncovered only the first surface region 10.5 while coveringthe rest of the first surface 10.3.

It will be appreciated that, with other embodiments of the presentinvention, the first surface region optically used may be smaller thanthe aperture provided by the cover.

Thus, the third area A₃ may be smaller than the second area A₂. Anyway,preferably, the third area A₃ corresponds to at least 80% of the secondarea A₂.

In other words, the aperture may leave uncovered a part of the firstsurface which is larger than the first surface region optically used andwhich is including the first surface region optically used. Anyway,preferably, the first surface region optically used corresponds to atleast 80% of the part of the first surface left uncovered by theaperture.

Furthermore, It will be appreciated that, with other embodiments of thepresent invention, depending on the dimensions of the ultimate opticalelement and the ultimate optical element holder, the aperture may evenleave uncovered the entire first surface such that the cover only coversat least a part of the ultimate optical element holder. With comparablylarge ultimate optical element holders, in particular, this may as wellprovide a sufficient relief of the ultimate optical element arrangementfrom stresses resulting from pressure differences between the firstspace and the second space.

Furthermore, It will be appreciated that, to achieve the abovebeneficial effects, with other embodiments of the present invention,depending on the rigidity distribution of the ultimate optical elementholder, the cover, at its outer circumference, instead of being mountedto the housing 3.1, may also be mounted to the ultimate optical elementholder. This may be done at a location where the unit formed by thehousing and the ultimate optical element holder connected thereto stillhas a rigidity which is sufficient that forces introduced by the coverinto the ultimate optical element holder do not substantially dislocatethe ultimate optical element. Depending on the design of the ultimateoptical element holder, such a location may, for example, be located atthe outer circumference of the ultimate optical element holder close tothe housing of the ultimate optical element unit.

The cover 14 is formed by a thin sheet metal. Anyway, it will beappreciated that, with other embodiments of the present invention, thecover may also be formed by one or more layers of metal and/or ofdifferent materials such as composite materials, plastics, ceramics,glass and any combination thereof.

The cover 14 has a rigidity in the direction of the optical axis 3.2which is sufficient to prevent direct contact of the cover 14 with thelast lens element 10 under any pressure difference between the firstspace 11 and the second space 12 that is to be expected during normaluse of the optical element unit 3. Anyway, the cover 14, within thelimitations stated above, may undergo deformations. Thus, the cover 14may be of very simple and lightweight design occupying relatively fewspace.

Since the cover 14 is a part mounted separately to the housing 3.1, thedistance between the last lens element 10 and the inner circumference ofthe cover may easily be adjusted to the specific requirements of therespective application. Furthermore, it will be appreciated that eitherone of the cover 14 and the last lens element 10 may be adapted ingeometry and surface properties, respectively, to provide appropriate oroptimized sealing boundaries within the gap between them.

Anyway, it will be appreciated that, with other embodiments of thepresent invention, the cover may also be formed monolithically with thepart of the housing provided by the last optical element module.

The sealing membrane 15 allows for relative motions between the cover 14and the last lens element 10 caused by such variations in the pressuredifference between the first space 11 and the second space 12.

It will be appreciated that, with other embodiments of the presentinvention, instead of the sealing membrane 15, another type of elasticsealing element or sealing substance, such as an elastic glue or thelike, may be located between the cover and the last lens element.Furthermore, a substantially rigid connection between the cover and thelast lens element may be chosen, provided that the cover in itself is ofsufficient rigidity in the direction of the optical axis so as to nottransfer a substantial part of the loads resulting from pressuredifferences between the first and second space onto the ultimate opticalelement.

In the following, a preferred embodiment of a method of manufacturingthe optical element unit 3 of FIG. 1 including a preferred embodiment ofa method of holding an optical element according to the presentinvention will be described with reference to FIGS. 1 to 4.

FIG. 4 shows a block diagram of a preferred embodiment of a method ofmanufacturing the optical element unit 3 of FIG. 1 including a preferredembodiment of a method of holding an optical element according to thepresent invention.

In a first step 16.1 the housing of the last optical element module 3.5is provided providing a part of the housing 3.1 and the inner housingpart 3.6 partly defining the first space 11 and the light passageway3.7.

In a second step 16.2, the last lens element 10 is provided and held inthe region of the light passageway 3.7 to provide a configuration as ithas been described above in the context of FIGS. 1 to 3.

In a third step 16.3, the first space 11 and the second space 12 areseparated by means of the cover 14 to provide a configuration as it hasbeen described above in the context of FIGS. 1 to 3. To this end thecover 14 is mounted in a pressure tight manner to the part of thehousing 3.1 the last optical element module 3.5 provides. Furthermore,the cover 14 is connected in a pressure tight manner to the last lenselement 10 via the sealing membrane 15 to provide a configuration as ithas been described above in the context of FIGS. 1 to 3.

Finally, in a fourth step, the other optical element modules 3.3 to 3.4are provided and the optical element modules 3.3 to 3.5 are assembled toform the optical element unit 3 in order to provide a configuration asit has been described above in the context of FIGS. 1 to 3.

It will be appreciated that the steps 16.1 to 16.3 form part of apreferred embodiment of a method of holding an optical element accordingto the invention. Furthermore, it will be appreciated that, with otherembodiments of the present invention, the above steps may be executed inany suitable different order to achieve the configuration as it has beendescribed above in the context of FIGS. 1 to 3.

Second Embodiment

In the following, a second preferred embodiment of an optical exposureapparatus 101 according to the present invention comprising an opticalprojection system 102 with an optical element unit 103 according to thepresent invention will be described with reference to FIGS. 5 and 6.

The optical exposure apparatus 101 is adapted to transfer an image of apattern formed on a mask 104 onto a substrate 105. To this end, theoptical exposure apparatus 101 comprises an illumination system 106illuminating the mask 104 and the optical element unit 103. The opticalelement unit 103 projects the image of the pattern formed on the mask 4onto the substrate 105, e.g. a wafer or the like.

To this end, the optical element unit 103 holds an optical element group107. This optical element group 107 is held within a housing 103.1 ofthe optical element unit 103. The optical element group 107 comprises anumber of optical elements 108 and optical elements 109 and 110, such aslenses, mirrors or the like. These optical elements 108, 109, 110 arealigned along an optical axis 103.2 of the optical element unit 103.

The optical projection system 102 receives the part of the light pathbetween the mask 104 and the substrate 105. Its optical elements 108,109, 110 cooperate to transfer the image of the pattern formed on themask 104 onto the substrate 105 located at the end of the light path. Tothis end, the optical elements 108, 109, 110 of the optical elementgroup 107 project the light received form the illumination system 106along the optical axis 103.2.

The optical element unit 103 is composed of a plurality of opticalelement modules 103.3 and optical element modules 103.4 and 103.5stacked and tightly connected to form the optical exposure unit 103.Each optical element module 103.3, 103.4, 103.5 holds one or more of theoptical elements 108, 109, 110, respectively.

The optical element modules 103.3, 103.4, 103.5 largely correspond tothe optical element modules 3.3, 3.4 and 3.5 of the optical element unit3 as shown in FIG. 2. Thus, it will here be mainly referred to thedifferences.

As may be seen from FIG. 6, the only difference with respect to theembodiment of FIGS. 1 to 3 lies within the fact that, instead of asealing membrane 15, a sealing gap 115 is formed between the innercircumference 114.2 of the cover 114 and the last lens element 110.

To avoid contamination of the first space 111 with contaminants from theenvironment surrounding the housing, the first pressure in the firstspace 111 is kept slightly superior to the second pressure in the secondspace 112 by a pressurizing unit 117 connected to the first space 111.Thus, a slight flow of the gas the first space 111 is filled with ismaintained through the sealing gap 115 towards the second space 112.

Apart from the described difference, the optical element modules 103.3,103.4, 103.5 are identical with the optical element modules 3.3, 3.4 and3.5 of the optical element unit 3 as shown in FIG. 2. Thus, for furtherdetails, it is here referred to the above description of the opticalelement unit 3.

It will be appreciated that, with other embodiments of the invention,instead of the pressure tight connection between the housing and thecover 14 and 114, respectively, a connection with a narrow sealing gapas described above may be provided.

Third Embodiment

In the following, a third preferred embodiment of an optical exposureapparatus 201 according to the present invention comprising an opticalprojection system 202 with an optical element unit 203 according to thepresent invention will be described with reference to FIGS. 7 to 9.

The optical exposure apparatus 201 is adapted to transfer an image of apattern formed on a mask 204 onto a substrate 205. To this end, theoptical exposure apparatus 201 comprises an illumination system 206illuminating the mask 204 and the optical element unit 203. The opticalelement unit 203 projects the image of the pattern formed on the mask 4onto the substrate 205, e.g. a wafer or the like.

To this end, the optical element unit 203 holds an optical element group207. This optical element group 207 is held within a housing 203.1 ofthe optical element unit 203. The optical element group 207 comprises anumber of optical elements 208 and optical elements 209 and 210, such aslenses, mirrors or the like. These optical elements 208, 209, 210 arealigned along an optical axis 203.2 of the optical element unit 203.

The optical projection system 202 receives the part of the light pathbetween the mask 204 and the substrate 205. Its optical elements 208,209, 210 cooperate to transfer the image of the pattern formed on themask 204 onto the substrate 205 located at the end of the light path. Tothis end, the optical elements 208, 209, 210 of the optical elementgroup 207 project the light received form the illumination system 206along the optical axis 203.2.

The optical element unit 203 is composed of a plurality of opticalelement modules 203.3 and optical element modules 203.4 and 203.5stacked and tightly connected to form the optical exposure unit 203.Each optical element module 203.3, 203.4, 203.5 holds one or more of theoptical elements 208, 209, 210, respectively.

FIG. 8 shows a schematic sectional representation of the last threeoptical element modules 203.3, 203.4 and 203.5 of the optical elementunit 203. As can be seen in particular from this Figure, the housing203.1 has an inner housing part 203.6 partly defining a first space 211and a light passageway 203.7 between the inner housing part 203.6 and asecond space 212 open to the environment surrounding the housing 203.1.While a first pressure prevails in the first space 211, a secondpressure prevails in the second space 212.

Within this light passageway 203.7, located at the exit end 203.8 of theoptical element unit 203, an ultimate optical element in the form of alast lens element 210 is held by an ultimate optical element holder inthe form of a last lens element holder 213 so as to be adjustable inposition. This last lens element 210 partly separates the first space211 and the second space 212 and, thus, partly defines the first space211 and the second space 212.

The last lens element 210 is a rotationally symmetric lens having afirst axis of symmetry 210.1 essentially coinciding with the opticalaxis 203.2 of the optical element unit 203. The last lens element 210mainly extends in a first plane perpendicular to the first axis ofsymmetry 210.1, as it is indicated in FIG. 8 by the dashed line 210.2.Furthermore, the last lens element 210 has a first perpendicularprojection onto the first plane 210.2 which has a first area A1.

The last lens element 210 has a first optical element surface in theform of a first lens surface 210.3 and a second optical element surfacein the form of a second lens surface 210.4. The first lens surface 210.3faces away from the penultimate optical element 209 of the opticalelement group 207. The second lens surface 210.4 faces towards thepenultimate optical element 209 of the optical element group 207.

The first lens surface 210.3 has a first surface region 210.5 opticallyused during an exposure process. This first surface region 210.5 is ofcircular shape. The outer circumference of the first surface region210.5 is indicated in FIG. 202 by the intersection of the dashed lines210.6 with the first lens surface 210.3.

On the side of the last lens element 210 facing away from thepenultimate optical element 209, a cover device in the form of a thinwalled cover 214 is provided. This cover 214 in its central partsubstantially has the form of a truncated conical shell. At its outercircumference 214.1, the thin walled cover 214 is mounted to the housing203.1 in a substantially pressure tight manner.

A third space 211.1 is defined by the housing 203.1, the last lenselement holder 213, the last lens element 210 and the cover 214. Thisthird space 211.1 communicates with the inner housing part 203.6 on theother side of the last lens element holder 213 via venting openings213.1 within the last lens element holder 213. Thus, the third space211.1 forms part of the first space 211, the first pressure alsoprevailing in the third space 211.1.

In this context, it will be appreciated that, with other embodiments ofthe present invention, the venting openings or venting passageways,additionally or alternatively, may also be provided in the housing andthe ultimate optical element, respectively.

The cover 214 extends between the housing 203.1 and the last lenselement 210. At its inner circumference 214.2, the cover 214 is locatedimmediately adjacent to the first lens surface 210.3, thereby forming anaperture 214.3 at this inner circumference 214.2. This aperture 214.3leaves a certain part of the first lens surface 210.3 uncovered by thecover 214.

The cover 214 is not contacting the last lens element 210 directly, ascan be seen from FIG. 9 in greater detail. In the embodiment shown, thecover 214 contacts the last lens element 210 via a sealing element inthe form of a thin sealing membrane 215. This sealing membrane 215extends between the inner circumference 214.2 of the cover 214 and thelast lens element 210. The sealing membrane 215 is connected to thecover 214 and the last lens element 210 in a pressure tight manner.

Thus, the first space 211 and the second space 212 are separated by thecover 214, the sealing membrane 215 and the last lens element 210 in apressure tight manner to avoid contamination of the first space 211 withcontaminants from the environment surrounding the housing.

The cover 214 is located such that a first part of the last lens element210 partly defines the first space 211 and that a second part of thelast lens element 210 partly defines the second space 212. Theperpendicular projection of this first part of the last lens element 210onto the first plane 210.2 is larger than the perpendicular projectionof this second part of the last lens element 210 onto the first plane210.2.

The cover 214 has the beneficial effect that only a second surfaceregion 210.7 of the first lens surface 210.3 is subjected to the secondpressure prevailing in the second space 212 while the rest of the lenssurface is subjected to the first pressure prevailing in the first space211. The outer circumference of this second surface region 210.7 isdefined by the sealing membrane 215 contacting the first lens surface210.3.

Thus, the force F acting in the direction of the optical axis 203.2 onthe last lens element 210 in the direction of the optical axis 203.2 dueto pressure differences between the first pressure p₁ in the first space211 and the second pressure p₂ in the second space 212 may be calculatedaccording to equation (1), i.e. as follows:

F=A _(ssr)·(p ₁ −p ₂)=A _(ssr) ·dp,

-   wherein: A_(ssr) is the area of the perpendicular projection of the    second surface region 210.7 onto the first plane 210.2;    -   dp is the pressure difference between the first pressure p₁ in        the first space 211 and the second pressure p₂ in the second        space 212.

Consequently, a variation Δdp in the pressure difference dp will resultin a variation ΔF in the force F which calculates according to equation(2), i.e. as follows:

ΔF=A _(ssr) ·Δdp.

As becomes apparent from equation (2), compared to a conventional designwithout a cover 214, the cover 214 considerably reduces the variation ΔFin the force F acting on the last lens element 210 due to a variationΔdp in the pressure difference dp between the first space 211 and thesecond space 212.

Thus, compared to a conventional design without a cover 214, at a givenpressure variation Δdp and a given rigidity of the last lens elementholder 213 in the direction of the optical axis 203.2 a lower positionvariation of the last lens element 210 due to this variation Δdp isachieved with the cover 214 according to the invention. Thus, the cover214 with the sealing membrane 215 forms a load relieving devicerelieving the last lens element 210 from loads resulting from pressuredifferences within the first space 211 and the second space 212.

As a consequence, to keep certain position variation limits for the lastlens element 210 during operation, less effort is required for therigidity of the last lens element holder 213 in the direction of theoptical axis.

It will be appreciated that, to achieve this beneficial effect accordingto the invention, the aperture 214.3 has a second perpendicularprojection onto the first plane 210.2 with a second area A₂ which issmaller than the first area A₁ of the first perpendicular projection ofthe last lens element 210 onto the first plane 210.2.

To largely take advantage of this beneficial effect without affectingthe optical performance of the optical system, the aperture 214.3 islocated as close as possible to the first surface region 210.5 opticallyused during an exposure process, as it is shown in the FIGS. 8 and 9.Thus, the second area A₂ substantially corresponds to of the thirdperpendicular projection of the first surface region 210.5 onto thefirst plane 210.2.

In other words, the aperture 214.3 is located as close as possible tothe first surface region 210.5 optically used during an exposure processsuch as to leave uncovered only the first surface region 210.5 whilecovering the rest of the first surface 210.3.

It will be appreciated that, with other embodiments of the presentinvention, the first surface region optically used may be smaller thanthe aperture provided by the cover. Thus, the third area A₃ may besmaller than the second area A₂. Anyway, preferably, the third area A₃corresponds to at least 80% of the second area A₂.

In other words, the aperture may leave uncovered a part of the firstsurface which is larger than the first surface region optically used andwhich is including the first surface region optically used. Anyway,preferably, the first surface region optically used corresponds to atleast 80% of the part of the first surface left uncovered by theaperture.

It will be further appreciated that, the embodiment of FIGS. 7 to 9allows for a greater relief of the last lens element 210 from loadsresulting from pressure differences within the first space 211 and thesecond space 212 than the embodiment shown in FIGS. 1 to 3. This is dueto the fact that the first surface region 210.5 optically used during anexposure process is smaller than the first surface region 10.5 opticallyused with the embodiment shown in FIGS. 1 to 3. This allows for ansmaller aperture 214.3 which is smaller than the aperture 14.3, leadingto a further reduction in the force F acting in the direction of theoptical axis 203.2 on the last lens element 210 due to pressuredifferences between the first space 211 and the second space 212.

The cover 214 is formed by a thin sheet metal. Anyway, it will beappreciated that, with other embodiments of the present invention, thecover may also be formed by one or more layers of metal and/or ofdifferent materials such as composite materials, plastics, ceramics,glass and any combination thereof.

The cover 214 has a rigidity in the direction of the optical axis whichis sufficient to prevent direct contact of the cover 214 with the lastlens element 210 under any pressure difference between the first space211 and the second space 212 that is to be expected during normal use ofthe optical element unit 203. Anyway, the cover 214, within thelimitations stated above, may undergo deformations. Thus, the cover 214may be of very simple and lightweight design occupying relatively fewspace.

Since the cover 214 is a part mounted separately to the housing 203.1,the distance between the last lens element 210 and the innercircumference of the cover may easily be adjusted to the specificrequirements of the respective application. Furthermore, it will beappreciated that either one of the cover 214 and the last lens element210 may be adapted in geometry and surface properties, respectively, toprovide appropriate or optimized sealing boundaries within the gapbetween them.

The sealing membrane 215 allows for relative motions between the cover214 and the last lens element 210 caused by such variations in thepressure difference between the first space 211 and the second space212.

It will be appreciated that, with other embodiments of the presentinvention, instead of the sealing membrane 215, another type of elasticsealing element or sealing substance, such as an elastic glue or thelike, may be located between the cover and the last lens element.Furthermore, a substantially rigid connection between the cover and thelast lens element may be chosen, provided that the cover in itself is ofsufficient rigidity in the direction of the optical axis so as to nottransfer a substantial part of the loads resulting from pressuredifferences between the first and second space onto the ultimate opticalelement.

It will be further appreciated that, with other embodiments of theinvention, instead of the sealing membrane 215, a sealing gap may beformed between the cover and the last lens element as it has beendescribed above in the context of FIGS. 5 and 6.

It will be further appreciated that, with other embodiments of theinvention, instead of the pressure tight connection between the housing203.1 and the cover 214, a connection with a narrow sealing gap asdescribed above in the context of FIGS. 5 and 6 may be provided.

It will be further appreciated that the optical element unit 203 ofFIGS. 7 to 9 may be manufactured with a method according to theinvention as it has been described above with reference to FIG. 4.

Fourth Embodiment

In the following, a fourth preferred embodiment of an optical exposureapparatus 301 according to the present invention comprising an opticalprojection system 302 with an optical element unit 303 according to thepresent invention will be described with reference to FIGS. 10 and 11.

The optical exposure apparatus 301 is adapted to transfer an image of apattern formed on a mask 304 onto a substrate 305. To this end, theoptical exposure apparatus 301 comprises an illumination system 306illuminating the mask 304 and the optical element unit 303. The opticalelement unit 303 projects the image of the pattern formed on the mask 4onto the substrate 305, e.g. a wafer or the like.

To this end, the optical element unit 303 holds an optical element group307. This optical element group 307 is held within a housing 303.1 ofthe optical element unit 303. The optical element group 307 comprises anumber of optical elements 308 and optical elements 309 and 310, such aslenses, mirrors or the like. These optical elements 308, 309, 310 arealigned along an optical axis 303.2 of the optical element unit 303.

The optical projection system 302 receives the part of the light pathbetween the mask 304 and the substrate 305. Its optical elements 308,309, 310 cooperate to transfer the image of the pattern formed on themask 304 onto the substrate 305 located at the end of the light path. Tothis end, the optical elements 308, 309, 310 of the optical elementgroup 307 project the light received form the illumination system 306along the optical axis 303.2.

The optical element unit 303 is composed of a plurality of opticalelement modules 303.3 and optical element modules 303.4 and 303.5stacked and tightly connected to form the optical exposure unit 303.Each optical element module 303.3, 303.4, 303.5 holds one or more of theoptical elements 308, 309, 310, respectively.

The optical exposure apparatus 301 further comprises a heat exchangeunit 317 for heating or cooling parts of the optical element unit 303.The heat exchange unit 317 comprises a cylindrical heat exchanger shell317.1 surrounding parts of the optical element unit 303 and a heatexchanging medium source 317.2 connected thereto. The cylindrical shell317.1 contains the ducts for distributing the heat exchanging mediumcoming from the heat exchanging medium source 317.2. In the embodimentshown, flow of a gaseous heat exchanging medium is established withinthe space between the heat exchanger shell 317.1 and the optical elementunit 303.

It will be appreciated that the heat exchanger shell 317.1 may only beassociated to a part of the optical element unit 303. Anyway, it is alsopossible that the cylindrical shell 317.1 envelopes the entire opticalelement unit 303. Furthermore, it will be appreciated that the heatexchange unit 317 may cool certain parts of the optical element unit 303while heating other parts of the optical element unit 303 at the sametime. To this end different loops of one or different heat exchangingmedia may be provided.

FIG. 11 shows a schematic sectional representation of the last threeoptical element modules 303.3, 303.4 and 303.5 of the optical elementunit 303. As can be seen in particular from this Figure, the housing303.1 has an inner housing part 303.6 partly defining a first space 311and a light passageway 303.7 between the inner housing part 303.6 and asecond space 312 open to the environment surrounding the heat exchangershell 317.1 and, thus, the housing 303.1. While a first pressureprevails in the first space 311, a second pressure prevails in thesecond space 312.

Within this light passageway 303.7, located at the exit end 303.8 of theoptical element unit 303, an ultimate optical element in the form of alast lens element 310 is held by an ultimate optical element holder inthe form of a last lens element holder 313 so as to be adjustable inposition. This last lens element 310 partly separates the first space311 and the second space 312 and, thus, partly defines the first space311 and the second space 312.

The last lens element 310 is a rotationally symmetric lens having afirst axis of symmetry 310.1 essentially coinciding with the opticalaxis 303.2 of the optical element unit 303. The last lens element 310mainly extends in a first plane perpendicular to the first axis ofsymmetry 310.1, as it is indicated in FIG. 11 by the dashed line 310.2.Furthermore, the last lens element 310 has a first perpendicularprojection onto the first plane 310.2 which has a first area A1.

The last lens element 310 has a first optical element surface in theform of a first lens surface 310.3 and a second optical element surfacein the form of a second lens surface 310.4. The first lens surface 310.3faces away from the penultimate optical element 309 of the opticalelement group 307. The second lens surface 310.4 faces towards thepenultimate optical element 309 of the optical element group 307.

The first lens surface 310.3 has a first surface region 310.5 opticallyused during an exposure process. This first surface region 310.5 is ofcircular shape. The outer circumference of the first surface region310.5 is indicated in FIG. 10 by the intersection of the dashed lines310.6 with the first lens surface 310.3.

On the side of the last lens element 310 facing away from thepenultimate optical element 309, a cover device in the form of a thinwalled cover 314 is provided. This cover 314 in its central partsubstantially has the form of a truncated conical shell. At its outercircumference 314.1, the thin walled cover 314 is mounted to the heatexchanger shell 317.1 via an open flange 317.3 of the heat exchangershell 317.1. Anyway, it will be appreciated that, with other embodimentsof the invention, the cover may also be mounted to any adjacent supportstructure other than the heat exchanger shell.

A third space 311.1 is defined by the housing 303.1, the last lenselement holder 313, the last lens element 310 and the cover 314. Thisthird space 311.1 communicates with the inner housing part 303.6 on theother side of the last lens element holder 313 via venting openings313.1 within the last lens element holder 313. Thus, the third space311.1 forms part of the first space 311, the first pressure alsoprevailing in the third space 311.1.

In this context, it will be appreciated that, with other embodiments ofthe present invention, the venting openings or venting passageways,additionally or alternatively, may also be provided in the housing andthe ultimate optical element, respectively.

A fourth space 312.1 is defined by the housing 303.1, the cover 314, theflange 317.3 and the heat exchanger shell 317.1. This fourth space 312.1communicates with the environment of the heat exchanger shell 317.1 viaventing openings within the flange 317.3. Thus, the fourth space 312.1forms part of the second space 312, the second pressure alsosubstantially prevailing in the fourth space 312.1.

The cover 314 extends between the heat exchanger shell 317.1 and thelast lens element 310. At its inner circumference 314.2, the cover 314is located immediately adjacent to the first lens surface 310.3, therebyforming an aperture 314.3 at this inner circumference 314.2. Thisaperture 314.3 leaves a certain part of the first lens surface 310.3uncovered by the cover 314.

The cover 314 is not contacting the last lens element 310. In theembodiment shown, similar to the embodiment shown in FIG. 6, a firstsealing gap 315.1 is formed between the inner circumference 314.2 of thecover 314 and the last lens element 310. Furthermore, a similar secondsealing gap 315.2 is formed between the housing 303.1 and the cover 314.

To avoid contamination of the first space 311 with contaminants from theenvironment surrounding the heat exchanger shell 317.1 and the housing313, respectively, the first pressure in the first space 311 is keptslightly superior to the second pressure in the second space 312. Thisis achieved by a pressurizing unit—not shown—connected to the firstspace 311. Thus, a slight flow of the gas the first space 311 is filledwith is maintained through the first sealing gap 315.1 and the secondsealing gap 315.2 towards the second space 312.

Both sealing gaps 315.1 and 315.2 have the advantage that axial andradial errors in positioning the cover 314 do substantially notinfluence the sealing performance and, in particular, do not influencethe positioning of the last lens element 310.

The cover 314 is located such that a first part of the last lens element310 partly defines the first space 311 and that a second part of thelast lens element 310 partly defines the second space 312. Theperpendicular projection of this first part of the last lens element 310onto the first plane 310.2 is larger than the perpendicular projectionof this second part of the last lens element 310 onto the first plane310.2.

The cover 314 has the beneficial effect that only a second surfaceregion 310.7 of the first lens surface 310.3 is subjected to the secondpressure prevailing in the second space 312 while the rest of the lenssurface is subjected to the first pressure prevailing in the first space311. The outer circumference of this second surface region 310.7 isdefined by the sealing membrane 315 contacting the first lens surface310.3.

Thus, the force F acting in the direction of the optical axis 303.2 onthe last lens element 310 in the direction of the optical axis 303.2 dueto pressure differences between the first pressure p₁ in the first space311 and the second pressure p₂ in the second space 312 may be calculatedaccording to equation (1), i.e. as follows:

F=A _(ssr)·(p ₁ −p ₂)=A _(ssr) ·dp,

-   wherein: A_(ssr) is the area of the perpendicular projection of the    second surface region 310.7 onto the first plane 310.2;    -   dp is the pressure difference between the first pressure p₁ in        the first space 311 and the second pressure p₂ in the second        space 312.

Consequently, a variation Δdp in the pressure difference dp will resultin a variation ΔF in the force F which calculates according to equation(2), i.e. as follows:

ΔF=A _(ssr) ·Δdp.

As becomes apparent from equation (2), compared to a conventional designwithout a cover 314, the cover 314 considerably reduces the variation ΔFin the force F acting on the last lens element 310 due to a variationΔdp in the pressure difference dp between the first space 311 and thesecond space 312.

Thus, compared to a conventional design without a cover 314, at a givenpressure variation Δdp and a given rigidity of the last lens elementholder 313 in the direction of the optical axis 303.2 a lower positionvariation of the last lens element 310 due to this variation Δdp isachieved with the cover 314 according to the invention. Thus, the cover314 forms a load relieving device relieving the last lens element 310from loads resulting from pressure differences within the first space311 and the second space 312.

As a consequence, to keep certain position variation limits for the lastlens element 310 during operation, less effort is required for therigidity of the last lens element holder 313 in the direction of theoptical axis.

It will be appreciated that, to achieve this beneficial effect accordingto the invention, the aperture 314.3 has a second perpendicularprojection onto the first plane 310.2 with a second area A₂ which issmaller than the first area A₁ of the first perpendicular projection ofthe last lens element 310 onto the first plane 310.2.

To largely take advantage of this beneficial effect without affectingthe optical performance of the optical system, the aperture 314.3 islocated as close as possible to the first surface region 310.5 opticallyused during an exposure process, as it is shown in FIG. 11. Thus, thesecond area A₂ substantially corresponds to of the third perpendicularprojection of the first surface region 310.5 onto the first plane 310.2.

In other words, the aperture 314.3 is located as close as possible tothe first surface region 310.5 optically used during an exposure processsuch as to leave uncovered only the first surface region 310.5 whilecovering the rest of the first surface 310.3.

It will be appreciated that, with other embodiments of the presentinvention, the first surface region optically used may be smaller thanthe aperture provided by the cover. Thus, the third area A₃ may besmaller than the second area A₂. Anyway, preferably, the third area A₃corresponds to at least 80% of the second area A₂.

In other words, the aperture may leave uncovered a part of the firstsurface which is larger than the first surface region optically used andwhich is including the first surface region optically used. Anyway,preferably, the first surface region optically used corresponds to atleast 80% of the part of the first surface left uncovered by theaperture.

It will be further appreciated that, the embodiment of FIGS. 10 and 11as well allows for a greater relief of the last lens element 310 and thelast lens element holder 313 from loads resulting from pressuredifferences within the first space 311 and the second space 312 than theembodiment shown in FIGS. 1 to 3. This is due to the fact that the firstsurface region 310.5 optically used during an exposure process issmaller than the first surface region 10.5 optically used with theembodiment shown in FIGS. 1 to 3. This allows for an smaller aperture314.3 which is smaller than the aperture 14.3, leading to a furtherreduction in the force F acting in the direction of the optical axis303.2 on the last lens element 310 due to pressure differences betweenthe first space 311 and the second space 312.

The cover 314 is formed by a thin sheet metal. Anyway, it will beappreciated that, with other embodiments of the present invention, thecover may also be formed by one or more layers of metal and/or ofdifferent materials such as composite materials, plastics, ceramics,glass and any combination thereof.

The cover 314 has a rigidity in the direction of the optical axis whichis sufficient to prevent direct contact of the cover 314 with the lastlens element 310 under any pressure difference between the first space311 and the second space 312 that is to be expected during normal use ofthe optical element unit 303. Anyway, the cover 314, within thelimitations stated above, may undergo deformations. Thus, the cover 314may be of very simple and lightweight design occupying relatively fewspace.

Since the cover 314 is a part mounted separately to the heat exchangershell 317.1, to the distance between the last lens element 310 and theinner circumference of the cover 314 may easily be adjusted to thespecific requirements of the respective application. Furthermore, itwill be appreciated that either one of the cover 314 and the last lenselement 310 may be adapted in geometry and surface properties,respectively, to provide appropriate or optimized sealing boundarieswithin the gap between them.

The sealing gaps 315.1 and 315.2 allow for relative motions between thecover 314 and the last lens element 310 caused by such variations in thepressure difference between the first space 311 and the second space312.

It will be further appreciated that, with other embodiments of theinvention, instead of the narrow sealing gaps 315.1 and 315.2, apressure tight connection, e.g. a sealing membrane, between the cover314 and the last lens element 310 and the housing 303.1, respectively,may be provided as it has been described above in the context of FIGS. 2and 3.

On the side of the cover 314 facing towards the last lens element 310there is mounted a set of circular first heat exchange medium lines317.4. These first heat exchange medium lines 317.4 form part of theheat exchange unit 317. They are fed with heat exchange medium via asecond heat exchange medium line 317.5 connected to the heat exchangingmedium source 317.2. Depending on the respective requirements of theapplication, the first heat exchange medium lines 317.4 may be fed withheating or cooling medium in order to heat or cool their surroundings.Of course, the heat exchange medium lines may also be integrated intothe cover.

It will be appreciated that, with other embodiments of the invention,the first heat exchange medium lines may also be located on the otherside of the cover. In general, it is preferred to locate such heatexchange medium lines on the side where the heat exchange effect isneeded most.

Furthermore, instead of providing separate heat exchange medium linesthe cover itself may be a part of the heat exchange unit by providing asufficient heat conduction towards and/or from the heat exchanger shell.To this end, elements enhancing heat transfer performance, such as heattransfer ribs or the like, may be provided on the cover instead of thefirst heat exchange medium lines 317.4.

It will be further appreciated that such a heat exchange unit may alsobe provided with any other embodiment of the invention having a covercovering part of the ultimate optical element arrangement, in particularcovering part of the ultimate optical element.

Finally, it will be appreciated that the optical element unit 303 ofFIGS. 10 and 11 may be manufactured with a method according to theinvention as it has been described above with reference to FIG. 4.

Fifth Embodiment

In the following, a fifth preferred embodiment of an optical exposureapparatus 401 according to the present invention comprising an opticalprojection system 402 with an optical element unit 403 according to thepresent invention will be described with reference to FIGS. 12 and 13.

The optical exposure apparatus 401 is adapted to transfer an image of apattern formed on a mask 404 onto a substrate 405. To this end, theoptical exposure apparatus 401 comprises an illumination system 406illuminating the mask 404 and the optical element unit 403. The opticalelement unit 403 projects the image of the pattern formed on the mask 4onto the substrate 405, e.g. a wafer or the like.

To this end, the optical element unit 403 holds an optical element group407. This optical element group 407 is held within a housing 403.1 ofthe optical element unit 403. The optical element group 407 comprises anumber of optical elements 408 and optical elements 409 and 410, such aslenses, mirrors or the like. These optical elements 408, 409, 410 arealigned along an optical axis 403.2 of the optical element unit 403.

The optical projection system 402 receives the part of the light pathbetween the mask 404 and the substrate 405. Its optical elements 408,409, 410 cooperate to transfer the image of the pattern formed on themask 404 onto the substrate 405 located at the end of the light path. Tothis end, the optical elements 408, 409, 410 of the optical elementgroup 407 project the light received form the illumination system 406along the optical axis 403.2.

The optical element unit 403 is composed of a plurality of opticalelement modules 403.3 and optical element modules 403.4 and 403.5stacked and tightly connected to form the optical exposure unit 403.Each optical element module 403.3, 403.4, 403.5 holds one or more of theoptical elements 408, 409, 410, respectively.

FIG. 13 shows a schematic sectional representation of the last threeoptical element modules 403.3, 403.4 and 403.5 of the optical elementunit 403. As can be seen in particular from this Figure, the housing403.1 has an inner housing part 403.6 partly defining a first space 411and a light passageway 403.7 between the inner housing part 403.6 and asecond space 412 open to the environment surrounding the housing 403.1.While a first pressure prevails in the first space 411, a secondpressure prevails in the second space 412.

Within this light passageway 403.7, located at the exit end 403.8 of theoptical element unit 403, an ultimate optical element in the form of alast lens element 410 is held by an ultimate optical element holder inthe form of a last lens element holder 413 so as to be adjustable inposition.

The last lens element 410 is a rotationally symmetric lens having afirst axis of symmetry 410.1 essentially coinciding with the opticalaxis 403.2 of the optical element unit 403. The last lens element 410mainly extends in a first plane perpendicular to the first axis ofsymmetry 410.1, as it is indicated in FIG. 13 by the dashed line 410.2.

The last lens element 410 has a first optical element surface in theform of a first lens surface 410.3 and a second optical element surfacein the form of a second lens surface 410.4. The first lens surface 410.3faces towards the penultimate optical element 409 of the optical elementgroup 407. The second lens surface 410.4 faces away from the penultimateoptical element 409 of the optical element group 407.

On the side of the last lens element 410 facing towards the penultimateoptical element 409, a sealing device in the form of a plane parallelplate 414 is provided within the inner part 403.6 of the housing 401.1.

A third space 412.1 is defined by the housing 403.1, the last lenselement holder 413, the last lens element 410 and the plane parallelplate 414. This third space 412.1 communicates with the environmentsurrounding the housing 403.1 on the other side of the last lens elementholder 413 via venting openings 413.1 within the last lens element toholder 413. Thus, the third space 412.1 forms part of the second space412, the second pressure also prevailing in the third space 412.1.

In this context, it will be appreciated that, with other embodiments ofthe present invention, the venting openings or venting passageways,additionally or alternatively, may also be provided in the housing andthe ultimate optical element, respectively.

At its outer circumference 414.1, the plane parallel plate 414 ismounted to the housing 403.1 in a substantially pressure tight manner.Thus, the first space 411 and the second space 412 are separated by theplane parallel plate 414 in a pressure tight manner to avoidcontamination of the first space 411 with contaminants from theenvironment surrounding the housing.

The plane parallel plate 414 is a translucent element made of glass.Anyway, it will be appreciated that, with other embodiments of thepresent invention, the sealing device may also only have a translucentsection in the region where useful light is crossing the location of thesealing device during an exposure process. In other words, thetranslucent section may be limited to the region optically used duringan exposure process. The outer circumference of such a translucentsection is indicated in FIG. 13 by the intersection of the dashed lines410.6 with the plane parallel plate 414, the dashed lines 410.6representing the region optically used during an exposure process.

The plane parallel plate 414 introduces a shift into the image producedby the lens elements 408, 409 and 410 of the optical element group 407.This has to be accounted for when designing the optical element group407. Thus, the plane parallel plate 414 represents a further opticalelement of the optical element group 407.

The plane parallel plate 414 is resiliently mounted to the housing, suchthat it may undergo excursions along the optical axis 403.2. In theembodiment shown, the mounting mechanism provides a parallel guide tothe plane parallel plate 414 such that it does not undergo any tiltingwith respect to the optical axis 403.2. Due to the change in volume itprovides, this resilient mounting mechanism also reduces the deformationof the plane parallel plate 414. Thus, the plane parallel plate 414 isoptically insensitive to such excursions along the optical axis 403.2,leading to substantially no alterations in the image produced by theoptical element group 407.

Anyway, it will be appreciated that, with other embodiments of theinvention, instead of a parallel guide, any other suitable mountingmechanism may chosen providing a corresponding support to the sealingdevice. In particular, the sealing device may simply be glued to thehousing with one or more layers of, preferably elastic, glue.

Furthermore, it will be appreciated that, with other embodiments of theinvention, instead of the plane parallel plate 414, any other suitableoptical element of different shape may be provided for forming thetranslucent section. It is only preferred that such an element issimilarly optically insensitive to excursions along the optical axis403.2, leading to substantially no alterations in the image produced bythe optical element group 407.

The plane parallel plate 414 has the beneficial effect that the lastlens element 410 and its holder 413 are not subjected to any loadsresulting from pressure differences between the first space 411 and thesecond space 412. These loads are completely taken by the plane parallelplate 414 substantially without affecting the quality of the imageproduced by the optical element group 407. Thus, the plane parallelplate 414 forms a load relieving device completely relieving the lastlens element 410 from loads resulting from pressure differences withinthe first space 411 and the second space 412.

As a consequence, to keep certain position variation limits for the lastlens element 410 during operation, considerably less effort is requiredfor the rigidity of the last lens element holder 413 in the direction ofthe optical axis.

As stated above, the plane parallel plate 414 is made of glass. Anyway,as also mentioned above, it will be appreciated that, with otherembodiments of the present invention, only a translucent section withinthe sealing device may be provided. This translucent section may be heldby a suitable holder. This holder may be made of metal and/or ofdifferent materials such as composite materials, plastics, ceramics,glass and any combination thereof.

It will be further appreciated that, with other embodiments of theinvention, instead of the pressure tight connection between the housing403.1 and the plane parallel plate 414, a connection with a narrowsealing gap as described above in the context of FIGS. 5 and 6 may beprovided.

It will be further appreciated that, with other embodiments of theinvention, in cases with very strict requirements with respect to therelief of the ultimate optical element arrangement, the fifth embodimentshown in the FIGS. 12 and 13 may be combined with one of the first tofourth embodiments previously described. In other words, the sealingdevice of the fifth embodiment may be combined with the cover device ofone of the first to fourth embodiments.

FIG. 14 shows a block diagram of a preferred embodiment of a method ofmanufacturing the optical element unit 403 of FIG. 12 including apreferred embodiment of a method of holding an optical element accordingto the present invention.

In a first step 416.1 the housing of the last optical element module403.5 is provided providing a part of the housing 403.1 and the innerhousing part 403.6 partly defining the first space 411 and the lightpassageway 403.7.

In a second step 4016.2, the last lens element 410 is provided and heldin the region of the light passageway 403.7 to provide a configurationas it has been described above in the context of FIGS. 12 and 13.

In a third step 416.3, the first space 411 and the second space 412 areseparated by means of the plane parallel plate 414 to provide aconfiguration as it has been described above in the context of FIGS. 12and 13. To this end the plane parallel plate 414 is mounted in apressure tight manner to the part of the housing 403.1 the last opticalelement module 403.5 provides. Furthermore, the plane parallel plate 414is connected in a pressure tight manner to the last lens element 410 toprovide a configuration as it has been described above in the context ofFIGS. 12 and 13.

Finally, in a fourth step 416.4, the other optical element modules 403.3to 403.4 are provided and the optical element modules 403.3 to 403.5 areassembled to form the optical element unit 403 in order to provide aconfiguration as it has been described above in the context of FIGS. 12and 13.

It will be appreciated that the steps 416.1 to 416.3 form part of apreferred embodiment of a method of holding an optical element accordingto the invention. Furthermore, it will be appreciated that, with otherembodiments of the present invention, the above steps may be executed inany suitable different order to achieve the configuration as it has beendescribed above in the context of FIGS. 12 and 13.

Sixth Embodiment

In the following, a sixth preferred embodiment of an optical exposureapparatus according to the present invention will be described withreference to FIGS. 1 and 15. The optical exposure apparatus, apart fromthe optical element unit 503, is identical with the optical exposureapparatus 1 described with reference to FIGS. 1 to 3. Thus, it will hereonly be referred to the differences relating to the optical element unit503.

The optical element unit 503 holds an optical element group. Thisoptical element group is held within a housing 503.1 of the opticalelement unit 503. The optical element group comprises a number ofoptical elements including optical elements 508, 509 and 510, such aslenses, mirrors or the like. These optical elements 508, 509, 510 arealigned along an optical axis 503.2 of the optical element unit 503.

The optical element unit 503 is composed of a plurality of opticalelement modules 503.3 and optical element module 503.4 stacked andtightly connected to form the optical exposure unit 503. Other than withthe first embodiment, the ultimate optical element module 503.5 isconnected to the penultimate optical element module 503.4 by a set ofactuators 517. Each optical element module 503.3, 503.4, 503.5 holds oneor more of the optical elements 508, 509, 510, respectively.

FIG. 15 shows a schematic sectional representation of the last threeoptical element modules 503.3, 503.4 and 503.5 of the optical elementunit 503. As can be seen in particular from this Figure, the housing503.1 has an inner housing part 503.6 partly defining a first space 511and a light passageway 503.7 between the inner housing part 503.6 and asecond space 512 open to the environment surrounding the housing 503.1.While a first pressure prevails in the first space 511, a secondpressure prevails in the second space 512.

Within this light passageway 503.7, located at the exit end 503.8 of theoptical element unit 503, there is provided an ultimate optical elementarrangement comprising an ultimate optical element in the form of a lastlens element 510 and an ultimate optical element holder in the form of alast lens element holder 513. The last lens element holder 513 isholding the last lens element 510. This last lens element 510 partlyseparates the first space 511 and the second space 512 and, thus, partlydefines the first space 511 and the second space 512. In thisembodiment, other than with the first embodiment, the last lens element510, together with the last lens element holder 513, is adjustable inposition via the actuators 517.

The last lens element 510 is a rotationally symmetric lens having afirst axis of symmetry 510.1 essentially coinciding with the opticalaxis 503.2 of the optical element unit 503. The last lens element 510mainly extends in a first plane perpendicular to the first axis ofsymmetry 510.1, as it is indicated in FIG. 2 by the dashed line 510.2.Furthermore, the last lens element 510 has a first perpendicularprojection onto the first plane 510.2 which has a first area A1.

The last lens element 510 has a first optical element surface in theform of a first lens surface 510.3 and a second optical element surfacein the form of a second lens surface 510.4. The first lens surface 510.3faces towards the penultimate optical element 509 of the optical elementgroup 507. The second lens surface 510.4 faces away from the penultimateoptical element 509 of the optical element group 507.

The first lens surface 510.3 has a first surface region 510.5 opticallyused during an exposure process. This first surface region 510.5 is ofcircular shape. The outer circumference of the first surface region510.5 is indicated in FIG. 15 by the intersection of the dashed lines510.6 with the first lens surface 510.3.

Between the last lens element 510 and the penultimate optical element509, a cover device in the form of a thin walled cover 514 is providedwithin the inner housing part 503.6. This cover 514 substantially hasthe form of a truncated conical shell. At its outer circumference 514.1,the thin walled cover 514 is mounted to the housing 503.1 in asubstantially pressure tight manner.

The cover 514 extends between the housing 503.1 and the last lenselement 510. At its inner circumference 514.2, the cover 514 is locatedimmediately adjacent to the first lens surface, thereby forming anaperture 514.3 at this inner circumference 514.2. This aperture 514.3leaves a certain part of the first lens surface 510.3 uncovered by thecover 514.

The cover 514 is not contacting the last lens element 510 directly butvia a sealing element in the form of a thin sealing membrane similar tothe sealing membrane 15 as it has been described in the context of thefirst embodiment. This sealing membrane extends between the innercircumference 514.2 of the cover 514 and the last lens element 510. Thesealing membrane 15 is connected to the cover 514 and the last lenselement 510 in a pressure tight manner.

Thus, the first space 511 and the second space 512 are separated by thecover 514, the sealing membrane 15 and the last lens element 510 in apressure tight manner to avoid contamination of the first space 511 withcontaminants from the environment surrounding the housing.

However, it will be appreciated that, with other embodiments of theinvention, instead of the sealing membrane, a sealing gap and a slightflow of gas as it has been described in the context of the secondembodiment (see in particular FIG. 6) may be used.

The cover 514 is located such that a first part of the last lens element510 partly defines the first space 511 and that a second part of thelast lens element 510 partly defines the second space 512. Theperpendicular projection of this first part of the last lens element 510onto the first plane 510.2 is smaller than the perpendicular projectionof this second part of the last lens element 510 onto the first plane510.2.

The cover 514 has the beneficial effect that only a second surfaceregion 510.7 of the first lens surface 510.3 is subjected to the firstpressure prevailing in the first space 511 while the rest of the lenssurface and also the last lens element holder 513 is subjected to thesecond pressure prevailing in the second space 512. The outercircumference of this second surface region 510.7 is defined by thesealing membrane 15 contacting the first lens surface 510.3.

Thus, the force F acting in the direction of the optical axis 503.2 onthe last lens element 510 in the direction of the optical axis 503.2 dueto pressure differences between the first pressure p₁ in the first space511 and the second pressure p₂ in the second space 512 may be calculatedaccording to equation (1), i.e. as follows:

F=A _(ssr)·(p ₁ −p ₂)=A _(ssr) ·dp,

-   wherein: A_(ssr) is the area of the perpendicular projection of the    second surface region 510.7 onto the first plane 510.2;    -   dp is the pressure difference between the first pressure p₁ in        the first space 511 and the second pressure p₂ in the second        space 512.

Consequently, a variation Δdp in the pressure difference dp will resultin a variation ΔF in the force F which calculates according to equation(2), i.e. as follows:

ΔF=A _(ssr) ·Δdp.

As becomes apparent from equation (2), compared to a conventional designwithout a cover 514, the cover 514 considerably reduces the variation ΔFin the force F acting on the last lens element 510 due to a variationΔdp in the pressure difference dp between the first space 511 and thesecond space 512.

Thus, compared to a conventional design with a conventional housing andwithout a cover 514, at a given pressure variation Δdp and a givenrigidity of the last lens element holder 513 in the direction of theoptical axis 503.2 a lower position variation of the last lens element510 due to this variation Δdp is achieved with the cover 514 accordingto the invention. Thus, the cover 514 with the sealing membrane 15 formsa load relieving device relieving the last lens element 510 and, thus,the ultimate optical element arrangement from loads resulting frompressure differences within the first space 511 and the second space512.

As a consequence, to keep certain position variation limits for the lastlens element 510 during operation, less effort is required for therigidity of, both, the last lens element holder 513 and the actuators517 in the direction of the optical axis 503.2.

It will be appreciated that, to achieve this beneficial effect accordingto the invention, the aperture 514.3 has a second perpendicularprojection onto the first plane 510.2 with a second area A₂ which issmaller than the first area A₁ of the first perpendicular projection ofthe last lens element 510 onto the first plane 510.2.

To largely take advantage of this beneficial effect without affectingthe optical performance of the optical system, the aperture 514.3 islocated as close as possible to the first surface region 510.5 opticallyused during an exposure process, as it is shown in FIG. 15. Thus, thesecond area A₂ substantially corresponds to of the third perpendicularprojection of the first surface region 510.5 onto the first plane 510.2.

In other words, the aperture 514.3 is located as close as possible tothe first surface region 510.5 optically used during an exposure processsuch as to leave uncovered only the first surface region 510.5 whilecovering the rest of the first surface 510.3.

It will be appreciated that, with other embodiments of the presentinvention, the first surface region optically used may be smaller thanthe aperture provided by the cover. Thus, the third area A₃ may besmaller than the second area A₂. Anyway, preferably, the third area A₃corresponds to at least 80% of the second area A₂.

In other words, the aperture may leave uncovered a part of the firstsurface which is larger than the first surface region optically used andwhich is including the first surface region optically used. Anyway,preferably, the first surface region optically used corresponds to atleast 80% of the part of the first surface left uncovered by theaperture.

Furthermore, It will be appreciated that, with other embodiments of thepresent invention, depending on the dimensions of the ultimate opticalelement and the ultimate optical element holder, the aperture may evenleave uncovered the entire first surface such that the cover only coversat least a part of the ultimate optical element holder. With comparablylarge ultimate optical element holders, in particular, this may as wellprovide a sufficient relief of the ultimate optical element arrangementfrom stresses resulting from pressure differences between the firstspace and the second space.

Furthermore, It will be appreciated that, to achieve the abovebeneficial effects, with other embodiments of the present invention,depending on the rigidity distribution of the ultimate optical elementholder, the cover, at its outer circumference, instead of being mountedto the housing 503.1, may also be mounted to the ultimate opticalelement holder. This may be done at a location where the unit formed bythe housing and the ultimate optical element holder connected theretostill has a rigidity which is sufficient that forces introduced by thecover into the ultimate optical element holder do not substantiallydislocate the ultimate optical element. Depending on the design of theultimate optical element holder, such a location may, for example, belocated at the outer circumference of the ultimate optical elementholder close to the housing of the ultimate optical element unit.

The cover 514 is formed by a thin sheet metal. Anyway, it will beappreciated that, with other embodiments of the present invention, thecover may also be formed by one or more layers of metal and/or ofdifferent materials such as composite materials, plastics, ceramics,glass and any combination thereof.

The cover 514 has a rigidity in the direction of the optical axis 503.2which is sufficient to prevent direct contact of the cover 514 with thelast lens element 510 under any pressure difference between the firstspace 511 and the second space 512 that is to be expected during normaluse of the optical element unit 503. Anyway, the cover 514, within thelimitations stated above, may undergo deformations. Thus, the cover 514may be of very simple and lightweight design occupying relatively fewspace.

Since the cover 514 is a part mounted separately to the housing 503.1,the distance between the last lens element 510 and the innercircumference of the cover may easily be adjusted to the specificrequirements of the respective application. Furthermore, it will beappreciated that either one of the cover 514 and the last lens element510 may be adapted in geometry and surface properties, respectively, toprovide appropriate or optimized sealing boundaries within the gapbetween them.

Anyway, it will be appreciated that, with other embodiments of thepresent invention, the cover may also be formed monolithically with thepart of the housing provided by the optical element module it is mountedto.

The sealing membrane allows for relative motions between the cover 514and the last lens element 510 caused by such variations in the pressuredifference between the first space 511 and the second space 512.

It will be appreciated that, with other embodiments of the presentinvention, instead of the sealing membrane 15, another type of elasticsealing element or sealing substance, such as an elastic glue or thelike, may be located between the cover and the last lens element.Furthermore, a substantially rigid connection between the cover and thelast lens element may be chosen, provided that the cover in itself is ofsufficient rigidity in the direction of the optical axis so as to nottransfer a substantial part of the loads resulting from pressuredifferences between the first and second space onto the ultimate opticalelement.

It will be appreciated that the optical element unit 503 may bemanufactured using the method of manufacturing an optical elementaccording to the present invention as it has been described above in thecontext of the first embodiment (i.e. with reference to FIGS. 1 to 4).

It will be further appreciated that, with other embodiments of thepresent invention, the covers described above in the context of thesecond to fifth embodiment may also be used together with an ultimateoptical element module that is connected to the penultimate opticalelement module by a set of actuators as it has been described above inthe context of the sixth embodiment.

Although, in the foregoing, embodiments of the present invention havebeen described where the ultimate optical element and the load relievingdevice are located at an exit end of an optical element unit, it will beappreciated that, with other embodiments of the present invention, theultimate optical element and the load relieving device may be located atany different location requiring a relief of an optical element fromloads resulting from pressure differences in the region of therespective optical element. Thus, the ultimate optical element and theload relieving device may be located at a transition area between theoptical element unit and a further optical device, such as a furtheroptical element unit. Furthermore, of course, the ultimate opticalelement and the load relieving device may be located at an entrance endof an optical element unit.

Furthermore, the present invention has been described in the context ofembodiments with rotationally symmetric optical components. Anyway, itwill be appreciated that the invention may also be used in the contextof any other optical element units comprising at least in partrotationally asymmetric components, in particular rotationallyasymmetric ultimate optical elements.

Furthermore, the present invention has been described in the context ofembodiments with an ultimate optical element that on both sides iscontacted by a gas atmosphere. Anyway, it will be appreciated that theinvention may also be used in the context of so called immersion systemswhere a part of the ultimate optical element is immersed in an immersionbath of an immersion medium located between the ultimate optical elementand the substrate, e.g. the wafer, to be worked.

Finally, the present invention has been described in the context ofembodiments for optical exposure processes. Anyway, it will beappreciated that the invention may also be used in the context of anyother optical application, where a relief of an optical element fromloads resulting from pressure differences in the region of therespective optical element is required.

1. (canceled)
 2. An optical element unit, comprising: an optical elementgroup configured to project light along an optical axis of the opticalelement group; a support structure comprising an inner part partlydefining a first space and a light passageway between the first spaceand a second space, the optical element group being disposed in theinner part; a cover device; and a holding device, wherein: the opticalelement group comprises first and second optical elements disposed in aregion of the light passageway, the first optical element has anoptically used surface; the second optical element has an optically usedsurface; the first optical element is connected to the support structureby the holding device; the cover device is a component separate from theholding device; the cover device is connected to the support structure;the cover device extends between the support structure and a firstsurface of the first optical element; the cover device is between thefirst and second optical elements; the cover device leaves a part of asurface of the first optical element uncovered; and the cover devicecovers a part of a surface of the first optical element.
 3. The opticalelement unit of claim 2, wherein an inner circumference of the coverdevice is located adjacent to the first optical element, and the innercircumference of the cover device defines an aperture.
 4. The opticalelement unit of claim 2, wherein the cover device has an innercircumference and an outer circumference, and the cover device isconnected to the support structure at the outer circumference.
 5. Theoptical element unit of claim 2, wherein: the first optical elementsurface has a first surface region which is optically used when usingthe optical element group to project light along the optical axis; andthe part of the surface of the first optical element which is leftuncovered by the cover device includes the first surface region of thefirst optical element.
 6. The optical element unit of claim 5, whereinthe part of the surface of the first optical element which is leftuncovered by the cover device is substantially identical to the firstsurface region of the first optical element.
 7. The optical element unitof claim 5, wherein the first surface region of the first optical devicecorresponds to at least 80% of the part of the first surface of thefirst optical element which is left uncovered by the cover device. 8.The optical element unit of claim 2, wherein the cover device is a thinwalled cover.
 9. The optical element unit of claim 2, wherein the coverdevice comprises a thin wall segment which extends between the supportstructure and the first optical element, and the thin wall segmentextends in a circumferential direction of the first optical element. 10.The optical element unit of claim 2, wherein the cover device is formedby one of a layer and a plurality of layers, each layer being made of amaterial selected from a material group, the material group comprising ametal material, a sheet metal material, a composite material, a plasticsmaterial, a ceramic material and a glass material.
 11. The opticalelement unit of claim 2, wherein the cover device does not directlycontact the first optical element.
 12. The optical element unit of claim2, wherein a gap is present between an inner circumference of the coverdevice and the first optical element.
 13. The optical element unit ofclaim 14, wherein the gap is configured to allow a definable continuousflow of a medium between the first space and the second space.
 14. Theoptical element unit of claim 2, wherein the cover device contacts thefirst optical element via a sealing device.
 15. The optical element unitof claim 2, further comprising an adjustment device configured to adjusta distance between the first optical element and the cover device. 16.The optical element unit of claim 2, wherein the a part of a surface ofthe first optical element faces the second optical element.
 17. Theoptical element unit of claim 2, wherein: the holding device comprises aholder which holds the first optical element within the supportstructure; the holder comprises a first side facing the cover device;the holder comprises a second side facing away from the cover device; athird space is defined on the first side of the holder between the firstoptical element holder and the cover device; a member provides at leasta first venting passageway towards the third space from the second sideof the first optical element holder; and the member comprises at leastone element selected from the group consisting of the support structure,the first optical element holder and the first optical element.
 18. Theoptical element unit of claim 2, wherein the cover device comprises aheat exchanging element which is connectable to a heat exchanging deviceto heat the cover device and/or cool the cover device.
 19. The opticalelement unit of claim 2, wherein the support structure comprises ahousing, and the first optical element is connected to the housing bythe holding device.
 20. An optical exposure apparatus configured totransfer an image of a pattern of a mask onto a substrate, the opticalexposure apparatus comprising: an optical element unit according toclaim 2, wherein the optical element unit is located within a light pathbetween the mask and the substrate.
 21. An optical element unitcomprising: an optical element group for projecting light along anoptical axis of the optical element group, a support structurecomprising an inner part partly defining a first space and a lightpassageway between the first space and a second space, the opticalelement group being in the inner part; and a cover device, wherein: theoptical element group comprises first and second optical element locatedin a region of the light passageway; the cover device is located betweenthe first and second optical elements; and the cover device is a thinwalled component comprising a truncated conical shell portion extendingbetween the support structure and a surface of the first opticalelement.
 22. The optical element unit of claim 21, wherein: a firstportion of the surface of the first optical element is uncovered by thecover device; and a second portion of the surface of the first opticalelement is covered by the cover device.
 23. An optical exposureapparatus configured to transfer an image of a pattern of a mask onto asubstrate, the optical exposure apparatus comprising: an optical elementunit according to claim 21, wherein the optical element unit is locatedwithin a light path between the mask and the substrate.